Petroleum recovery by thermal backflow



Nov. 5, 1968 M. PRATS PETROLEUM RECOVERY BY THERMAL BACKFLOW Filed June 9, 1967 INVENTOR:

MICHAEL PRATS R IQWEM HIS AGENT United States Patent 3,409,083 PETROLEUM RECOVERY BY THERMAL BACKFLOW Michael Prats, Houston, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 421,294, Dec. 28, 1964. This application June 9, 1967, Ser. No. 645,030

1 Claim. (Cl. 166-40) ABSTRACT OF THE DISCLOSURE An improved backflow process for recovery of oil from an-underground formation that contains both oil and water comprising establishing communication between a well borehole and said formation, thermal soaking the oil in the recovery zone by injecting steam and initiating an underground combustion therein to form non-condensable gas and additional steam and thereafter reducing the pressure within the well borehole to less than the formation pressure to allow fluid inclusive of oil to backflow into the well borehole.

Cross-references to related applications This application is a continuation-in-part of copending application Ser. No. 421,294, filed Dec. 28, 1964, now US. Patent No. 3,333,637.

Background of the invention The invention relates broadly to the recovery of petroleum from subterranean reservoirs by thermal backflow, and more particularly to a method of backflow recovering petroleum from underground formations wherein thermal soaking is accomplished by injecting steam and also initiating an underground combustion by injecting an oxygencontaining gas into the reservoir formation. The combustion products contain non-condensable fluids and these fluids as well as the combustion-produced steam are utilized in the recovery of petroleum by backflow techniques of the present invention. The primary source of the reservoir-heating fluid can come from the formation of steam in situ.

It is known that in many petroleum-bearing reservoirs only a small portion of the petroleum is recovered by primary recovery techniques which depend on natural gas or water pressure and/or gravity in the reservoir system to flow the petroleum into the borehole. When the pressure drops because of the removal of some of the petroleum or the petroleum is too viscous to be removed by these natural pressures in the reservoir system, primary recovery (natural flow) is not possible. Since there are substantial quantities of petroleum remaining in the reservoir in both such situations, supplemental recovery techniques are often employed to recover additional petroleum from the reservoir.

These supplemental techniques are sometimes referred to as secondary recovery methods and'a useful dichotomy of these secondary methods are those involving drives, such as waterfloods, and those involving backflow where the injection and recovery are accomplished from the same individual borehole. In the case of drives spaced wells are used and the petroleum is driven by injecting fluid into the reservoir at one location to create a pressure differential that causes fluid in the reservoir to move into a spaced recovery location. In backflow techniques, after fluid has been injected into the reservoir, the injected fluid and the petroleum is backflowed into the same well at the same location that was used for the injection of the fluid.

Each of the backflow and drive techniques has certain advantages over the other. The drive techniques have been generally preferred since they are usually more efficient in that the fluid is continually moved in the direction, of the gradient that is first established within the reservoir; whereas, in the backflow techniques, the injected fluid must first bypass the oil and then displace the oil back into the opening through which the fluid was injected. The principal disadvantage of drives are the expense of drilling of additional wells, and locating such wells so that they encounter the same reservoir and the space between the wells is free of discontinuities caused by impermeable streaks, faulting and the like which would render the drives inoperable. While the backflow techniques avoid the abovementioned disadvantages of the drives, in many instances the conventional backflow techniques have not been effective enough to offset the additional oil recovery obtained by drives. In general, the conventional backflow techniques are more eflicient when they employ heated fluids in reservoirs containing an oil having a viscosity such that the hot fluid tends to bypass much of the petroleum around the well, but in doing so heats it to a temperature at which it may be more easily displaced into the well during the backflowing of the well. This beneficial effect tends to be lost where the oil viscosity is initially low enough to allow a large proportion of the oil to be displaced away from the Well as the hot fluid is injected or where the hot fluid tends to flash-off light hydrocarbons and leave a viscous petroleum residue in the formation. In the latter situation the viscous residue may tend to plug some of the pores of the reservoir adjacent to the borehole and make the backflow more diflicult.

Summary of the invention In the process of the parent application backflow recovery of petroleum is accomplished by injecting a slug of non-condensable gas through a borehole into a ie troleum-bearing reservoir, subsequently injecting a heated condensable fluid, then producing a fluid from the reservoir by reducing the ressure in the borehole to less than the pressure in the reservoir and recovering the petroleum that flows into the borehole.

The present invention is an extension or modification of the parent backflow process for petroleum recovery in respect to recovering petroleum from earth formations which contain both oil and water. The present process comprises:

1) establishing fluid communication between a well and a portion of an earth formation which contains oil and water;

(2) heating a portion of the earth formation in the presence of oxygen and steam and initiating an underground combustion of the oil;

(3) injecting oxygen-containing gas through the well at a rate maintaining the combustion and displacing combustion products and steam into the earth formation beyond the combustion zone; and,

(4) reducing the pressure within the well to less than that within the earth formation beyond the combustion zone and producing oil from the well.

Description of preferred embodiments The specified injection into an earth formation containing oil and water is not limiting from a practical point of view, since most oil-bearing earth formations contain suflicient water, usually to connate form.

The advantages of the present invention relative to conventional steam drive'or underground combustion drive processes are essentially those of a backflow type of production as distinguished from a drive between a pair of wells.

Although the backflow process of the parent application, i.e., using injection of non-condensable gas .followed by injection of heated condensable fluids such as steam, is generally effective and economical in recovering petroleum, the process of the present invention offers additional advantages by providing 1) a means of effecting a steam soak type of production process in a location in which the water availability situation makes it undesirable to generate all of the steam at a surface location and (2) an attractive means of extending the depth of penetration obtainable in a steam soak operation, particularly when applied after a first cycle in which the injected fluid was steam or steam preceded by a slug of non-condensable gas.

Any suitable means for initiating in situ combustion of the oil in underground formations that contain oil and water can be used. The earth formation is preferably preheated with steam to a moderate temperature and ig nition is preferably obtained by injecting steam containing at least about 20 percent air and utilizing the heat of the oil oxidation to supplement the heat needed to attain the ignition temperature. Where an earth formation has been previously heated by a prior injection backflow cycle as is, for example, described in the parent case, ignition can be attained by simply injecting a combustion-supporting gas such as oxygen, air, etc. In the former case spontaneous ignition in the formation can be achieved by use of a mixture of steam and air containing at least 20 percent by weight of air as the initiating combustion fluid. Instead of injecting a mixture of steam and oxygen-containing gas, the two fluids can be injected alternatively.

Once combustion has been initiated, the combustion zone can be advanced by injection of a mixture of air and water. The water injection helps control the temperature in the combustion front and controls or restricts cracking and coking as well as enhancing the depth of penetration of the heat zone thereby advantageously increasing the amount of oil that can be heated within a given time.

In general, after some steam has been injected, for example, to preheat, the oil for ignition and/or as a steam soak cycle, where the recovery zone being heated contains both oil and water, then the combustion front can advantageously be advanced by conventional dry procedures and/or by a procedure in which the temperature is controlled by limiting the quantity of the injected oxidizing gas used to support the combustion. Normally in a dry combustion process where water (connate) is present in the oil-bearing formation, an oxidizing gas is injected into the formation after a portion of the hydrocarbons in the formation has been raised to ignition temperature resulting in establishment of a combustion front which in time establishes in .front of it a heated liquid condensate zone due to the presence of water which is vaporized and recondensed in the cooler areas of the formation. Also, some of the combustible gases which are formed during the combustion finger pass the condensated liquid zone and establish a non-condensable gaseous zone essentially as desired in the parent appli cation. The combustion front may be controlled by limiting the quantity of injected oxidizing gas. The water formed by selective combustion of the hydrogen atoms from the residual hydrocarbons in the formation together with the water present in the pores of the formation is displaced in vapor form by means of the combustion gases away from the well bore. As the process proceeds temperature of the vapor drops, it condenses and the heat emitted there-from heats oil-bearing zone.

In general, the oxygen-containing gas which is injected in order to initiate and maintain the combustion provides sufficient non-condensable gas to enhance a steam soak, by providing a gas-aided steam soak of the type described in the parent case. Once the combustion is initiated, it is maintained for a period of from several weeks to several months. The injection pressures are preferably kept below those corresponding to the weight of the overburden.

In the present process, the fluids which are injected tend to become distributed within the reservoir formation in the manner described in the parent case. During a given injection cycle, a localized pressurized zone is created within the portion of the reservoir formation in which fluid has time to move during the period in which fluid is being injected. The gaseous fluids which are injected or formed within the formation, being more mobile than liquids, tend to flow around and bypass much of the reservoir oil while displacing most of the liquid water further into the reservoir formation. The heated gases and steam give up their heat to the liquid and solid components of the reservoir formation and the steam condenses to become a liquid that is also bypassed by the non-condensable gases. The resulting fluid is distributed as illustrated by FIGURE 1.

Specifically, the figure is a vertical section through an earth formation showing a borehole traversing multiple strata and a reservoir wherein is illustrated the conditions encountered during the practice of the invention.

Brief description of the drawing Referring to the drawing a borehole 11 penetrated from the surface through overburden strata 12 and a petroleum producing reservoir 13 which is sandwiched between impermeable layer 14. The reservoir, though shown as a horizontal one, could have a reasonable degree of dip without effecting the practice of this invention.

A well casing 15 is shown secured in the borehole 11 with a sealant 16, such as cement, and in the area of the vertical traverse of the reservoir 13 the casing string 15 and sealant 16 are perforated with ports 17 to provide fluid communication with the inside of the casing string 15 and the reservoir 13.

At the top of easing string 15 a gland 18 secures the outer pipe string 19, of two concentric pipe strings 19 and 20, respectively, which extend downwardly inside the casing string. The inner pipe string 20 preferably extends to the bottom of the casing string 15, extending below the end of the outer string 19. The annulus between the easing string 15 and the inner pipe string 20 is sealed with packer 23 in the lower portion of the reservoir 13 so that there is a lower compartment 21 through which only the inner pipe string 20 has fluid communication with the reservoir 13. The annulus between the outer pipe string 19 and the casing string 15 is sealed with packer 22 near the top of reservoir 13 and provides an upper compartment 21a through which the outer pipe string 19 has fluid communication with the reservoir 13.

In this arrangement the lower compartment 21 communicating with the lower reservoir serves as a sump during backflow of petroleum to the borehole and either or both of the compartments can be used for injection. Such an arrangement is desirable in a reservoir in which there will be a significant gravity segregation of the fluids in the reservoir and/or where there is a difference in the permeabilities of the upper and lower portions of the reservoir, etc. This sump can also be used to prevent portions of the gaseous slug from moving back toward the borehole and venting prematurely into the casing string 15 by closing off pipe string 19 and recovering only through pipe string 20.

In general, the arrangement of the well conduits for injecting fluids into and producing fluids from the reservoir formation should be designed for the optimum recovery from the particular formation being treated.

For a better understanding of the invention the oil recovery reservoir 13 is divided into zones A, B, C and D to show the conditions in the reservoir during the practice of this invention. As an oxygen-containing gas such as oxygen or air or mixtures of such gases and a fluid such as water is injected into the formation through the well borehole via tubing strings 19 and/ or 20 to cause initiation of a combustion front, the upstream portion of zone A becomes an oil-depleted zone through which a combustion-supporting fluid is conveyed to a combustion front bordering on zone B. In zone B a condensation front is established which comprises of a partially heated zone containing oil, condensate, steam and a small amount of gas. As the injection of oxygen-containing gas through the well is maintained at a rate to support combustion in the oil reservoir, the non-condensable combustion products are displaced into zone C. The zone D is a pressurized zone in which the formation fluid pressure becomes increased for the duration of the process due to the injection of fluid as described into the formation. The pressurized zone D enables the backflow of oil into the well conduits when pressure adjustments are made such as by lowering the pressures in the casing string 15 below a pressure existing between the borehole and the outer boundary of zone D. Further, the petroleum in bank B is heated more as it moves into the areas adjacent to the borehole where temperatures are higher which improves the mobility ratio still further.

Also, it may be desirable to reduce the pressure in borehole 11 after the residual pressures have been released by placing a partial vacuum thereon.

The method of the invention is especially effective when the petroleum does not originally contain high proportions of dissolved gas. Further, it should be appreciated that some steam will finger through the petroleum bank and aid the gaseous slug as a drive mechanism during backfiow. As the pressure drops in the casing 15, the gaseous slug sweeps toward the casing string 15 and the partial pressure of steam will be reduced and some of the water condensate will be flashed into steam to aid the drive mechanism in forcing the petroleum bank toward the borehole 11.

I claim as my invention:

1. A method of recovering petroleum from a subter ranean reservoir by a thermal backflow which comprises the steps of:

establishing fiuid communication between a single well borehole and a reservoir that contains oil and water;

heating a portion of said reservoir in the presence of steam and oxygen-containing gas and initiating underground combustion of the oil;

injecting a combustion supporting mixture of water and oxygen-containing gas through the well at a rate sufficient to maintain combustion;

displacing gaseous combustion products and steam into the formation beyond the combustion zone;

terminating said injection of combustion supporting mixture of water and oxygen-containing gas and a1- lowing the formation to thermally soak;

reducing the pressure within the well borehole to less than the pressure within the formation and producing oil from the well by backflow; and, repeating the heating, injecting, displacing, terminating and pressure reducing steps to produce additional amounts of oil.

References Cited UNITED STATES PATENTS 3,129,757 4/1964 Sharp 166-11 3,139,928 7/ 1964 Broussard 1662 3,171,482 3/1965 Simm 166-39 3,259,186 7/1966 Dietz 166-11 3,285,336 11/1966 Gardner 166-11 3,292,702 12/1966 Boberg 166-40 3,330,353 7/1967 Flohr 166-2 X 3,332,482 7/1967 Trantham 166-2 STEPHEN J. NOVOSAD, Primary Examiner. 

